Joint prostheses

ABSTRACT

Joint prostheses are disclosed which include a substantially smooth male portion (e.g. a femoral cap or a trochlea) and a mated female portion (e.g. a tibial plateau or an olecrenon). Prosthesis forms may be prepared by forming initially the male portion, pressing the male portion into molding material to initiate the formation of the female portion, and passing the portions through flexion in contact to form flexion surfaces on the female portion. The components of the present prostheses are suitable as several joint prostheses, such as knee, elbow, ankle, shoulder, finger and toe joints, including a first component of an inert metal alloy and a second component of molded plastic having at least one flexion surface and upwardly extending guiding-bearing surfaces surrounding same. Prosthesis forms may be prepared similarly, including the formation of a flexion surface by passing the joint through flexion. A knee prosthesis is described including a substantially smooth femoral cap in combination with a mated tibial plateau. The tibial plateau is formed to include means to lock the femoral cap and tibial plateau in extension and means to constantly change the instantaneous centers of rotation during flexion. The tibial plateau surfaces are such that there is maximal surface contact at extension between the tibial plateau and the femoral cap, and during flexion there is substantial surface contact. An elbow prosthesis is described including a substantially smooth humeral cap in combination with a mated ulnar component. The ulnar component is formed to include mating surfaces corresponding to the olecrenon and coronoid processes, as well as the radial head if desired. During flexion of the elbow joint, there is substantial surface contact between the humeral cap and the surface of the ulnar component.

United States Patent [191 Ewald 1 JOINT PROSTHESES [76] Inventor: Frederick C. Ewald, 916

Greenwood Avenue, Winnetka, 111. 60093 22 Filed: July 9,1971

21 Appl.No.: 161,086

[52] US. Cl. 3/1, 128/92 C [51] Int. Cl. A6lf l/24 [58] Field of Search 3/1, 22; 128/92 C, 92 CA, 128/92 R [56] References Cited UNITED STATES PATENTS 3,694,821 10/1972 Moritz 3/1 3,696,446 10/1972 Bousquet et al. 3/1

3,506,982 4/1970 Steffee 3/1 FOREIGN PATENTS OR APPLICATIONS 1,047,640 7/1953 France 128/92 C OTHER PUBLICATIONS Ossacryl Endo Prostheses" (Advertisement) page 51, Journal of Bone & Joint Surgery, July 1952. Vitallium Surgical Appliance (Catalog), Austenal Medical Div. Howmet Corp., N.Y., NY. 1964, page 62. FIGS. No. 6662 and 6958-6961 Relied upon.

Primary Examiner-Richard A. Gaudet Assistant Examiner-Ronald L. Frinks [57] ABSTRACT Joint prostheses are disclosed which include a substantially smooth male portion (e.g. a femoral cap or a 1 Mar. 26, 1974 trochlea) and a mated female portion (e.g. a tibial plateau or an olecrenon). Prosthesis forms may be prepared by forming initially the male portion, pressing the male portion into molding material to initiate the formation of the female portion, and passing the portions through flexion in contact to form flexion surfaces on the female portion. The components of the present prostheses are suitable as several joint prostheses, such as knee, elbow, ankle, shoulder, finger and toe joints, including a first component of an inert metal alloy and a second component of molded plastic having at least one flexion surface and upwardly extending guiding-bearing surfaces surrounding same. Prosthesis forms may be prepared similarly, including the formation of a flexion surface by passing the joint through flexion. A knee prosthesis is described including a substantially smooth femoral cap in combination with a mated tibial plateau. The tibial plateau is formed to include means to lock the femoral cap and tibial plateau in extension and means to constantly change the instantaneous centers of rotation during flexion. The tibial plateau surfaces are such that there is maximal surface contact at extension between the tibial plateau and the femoral cap, and during flexion there is substantial surface contact.

An elbow prosthesis is described including a substantially smooth humeral cap in combination with a mated ulnar component. The ulnar component is formed to include mating surfaces corresponding to the olecrenon and coronoid processes, as well as the radial head if desired. During flexion of the elbow joint, there is substantial surface Contact between the humeral cap and the surface of the ulnar component.

10 Claims, 20 Drawing Figures PATENTEDHARZG m4 slvselsvs SHEET 2 OF 4 PATENTEDMARZS m4 3398,67'9

SHEET a [1F 4 1 JOINT PROSTHESES BACKGROUND OF THE INVENTION An objective of the present invention is a total replacement prosthesis for the knee joint, elbow joint or other joints of the body. The present invention has a preferred application as a knee prosthesis, in part because of the relative complexity of the joint; hence this application will be directed principally to that embodiment. While advances have been made in the last decade in the state of the art, less than satisfactory results have been obtained, particularly with the knee joint prostheses now available. Knee prostheses which have been put in use include the Young mechanical knee, the Walldius mechanical knee, MacIntosh tibial plateaus, M.G.H. femoral condyle replacements, McKeever tibial plateaus, Shiers knee prosthesis and Townley tibia plateau plates. (Other similar joint prostheses include the well-known Charnley hip joint, as well as the Zimmer shoulder prosthesis.)

The existing knee joint prostheses designed for replacement of the total joint have in common a metal hinge and intramedullary stems for anchoring to the femur and tibia. Because of the complexity of the knee joint action, however, such prostheses apply considerable stress to the hinge portions. This stress causes wear to the joint, which can result in dispersion of metal into surrounding tissue with consequent complications, and in weakening or failure of the hinge joint itself. These problems can be minimized by the use of particular alloys in the construction of the hinge joint, but, in that a hinge by its very nature permits rotation only through a single plane, it cannot duplicate the complex movements of the knee joint, and a less than satisfactory result is inevitable. See, e.g., D.V. Girzadas et al., Performance of a Hinged Metal Knee Prosthesis, J. Bone and Joint Surgery, Vol. 50-A No. 2, March 1968, pp. 355 et seq.

Recent activities of my colleagues have indicated a desire to eliminate the hinge from the knee joint prosthesis. Though these efforts have been intense, it is recognized that a completely satisfactory joint has still not been achieved, particularly because of the very complex motions of the component parts of the knee joint during flexion, and the requirement that the joint be locked when in full extension. The resulting tibial plateau and femoral cap combinations not only permit movement of the knee joint during extension but also are incapable of duplicating flexion. Thus, while it has been found possible through the use of tibial plateau and femoral cap combinations to obtain a more satisfactory prosthesis, there still remains considerable room for improvement in this area. In particular, it appears that no existing prosthesis obtains at once the rotation of the tibia with respect to the femur, the pivoting of the medial condyle about the lateral condyle and the translation of the femure with respect to the tibia-a movement in different planes at once (sagittal and transverse) which is achieved during flexion of the normal knee joint.

It is therefore a major objective of this invention to provide both strength and stability during extension and flexion of the knee joint prosthesis, while at the same time permitting control and guide mechanisms during flexion as nearly like those in a normal knee joint as possible. A particular objective is to obtain a prosthesis which permits at once the motions of naturethe femure-tibia rotation and translation and the medial-lateral condyle pivoting. By the attainment of such objectives, not only will a suitably stable knee joint prosthesis be provided, but also during use it will come as close as possible to permitting normal joint action.

In the normal action of a knee joint, motion of the tibia on the femur is guided by a complex combination of collateral ligaments, cruciate ligaments, and menisci, and is controlled by the thigh muscles. As a consequence, however, of disease in the knee joint to the extent of amputation, the menisci and cruciate ligaments are usually necessarily destroyed, thereby leaving only the collateral ligaments to perform the guiding function. Therefore, in order to simulate the normal guiding of the joints during flexion, a knee prosthesis would require means which operate, together with the retained collateral ligaments, like the cruciate ligaments and menisci components of the normal joint. It is therefore a further objective of this invention to provide a knee joint prosthesis capable of performing the functions of these two components. The function of these components is of course to guide the tibia in its tracks on the femoral condyles in a normal stable pattern of movement, which is controlled by the muscles of the thigh.

This movement, as mentioned above, involves the femur-tibia rotation and translation and medial-lateral condyle pivoting, simultaneously and in combination. For a detailed description of the control mechanism and the guiding components of the knee joint during normal extension and flexion see O.C. Brantigan et al., The Mechanics of the Ligaments and Menisci of the Knee Joint, J. Bone and Joint Surgery, Vol. XXIII,

No. 1, January 1941, pp. 44 et seq.; and A.J. Helfet,

Control and Guide Mechanism of the Knee Joint, A.A.O.S. Instructional Course Lecures (1970), pp. 64-65.

The principles and objectives which apply particularly to the knee joint have at least partial application to other joints as well. I have found, for example, that none of the human joints operates precisely as a hinge but all involve to a degree more complex motion. Accordingly, the prostheses of this invention, which do not rely on a hinged joint mechanism, are superior to those that do. Moreover, the articular surfaces of the prosthesis of the present invention obtain the advantages of the hinge joint without its concomitent restrictions. Further, the articular surfaces of this invention, which are particularly important in providing the functions of guidance, stability and connection otherwise absent because of the removal of one or both cruciate ligaments in the knee, are similarly important in other prostheses, for joints where in most cases one or more collateral ligaments are lost during disease or surgery.

BRIEF SUMMARY OF THE INVENTION The joint prostheses of the invention include a substantially smooth male portion and a mated female portion. In general, the male portion is similar in shape to the end of the bone which it replaces, although usually somewhat smaller and smoother. The female portion is considerably different in shape from the end of the other bone of the joint, since it includes articular surfaces which function to provide the control and connection of the normal joint, by mating with the male surface during flexion. In the knee joint these articular surfaces perform the functions of the cruciate ligaments and menisci; in the elbow or other joints, the articular surfaces likewise function as the capsular ligaments which may have been destroyed by disease or injury or necessarily removed during prosthetic replacement.

The knee prosthesis of the present invention comprises in combination two components: a femoral cap and a tibial plateau. These components are mated, preferably by the method herein described, so that they operate in conjunction to permit normal knee-joint functioning. In particular, the tibial plateau comprises articular surfaces, described in greater detail hereinafter, which operte in conjunction with the femoral cap to perform the functions of the cruciate ligaments and menisci in the normal knee.

The femoral cap is similar in shape to the normal distal end of the femur, although somehat smaller and smoother. A model for the femoral cap may be formed by cutting at least one-eighth inch from the outer surfaces of a substantially life-size femoral cap, while forming medial and lateral condyles, substantially smooth and round in shape. While uniform in contour, the surface of the cap may be highly polished or rough, or even perforated. The femoral cap is preferably constructed of an inert metal alloy-stainless steel, cobaltchromium alloy, for example, those sold under the trademarks VITALLIUM, or ZIMALLOY, or a titanium alloy being suitable and preferred. The cap may be formed by molding molten, softened or powdered alloy metal, or by carving or otherwise shaping the metal or other material. If desired, particles of hard material may be incorporated on the cap surface to improve wear-resistance, or lubricants may be added, by known methods.

The tibial plateau component of the present combination is considerably different from the proximal end of the tibia. Part of the reason for this difference is the objective that the component, in conjunction with the femoral cap, perform the function of the cruciate liga ments and menisci in the original knee joint. Thus, the tibial plateau includes means to lock the femoral cap and tibial plateau in extension; and means, permitting the normal femur-tibia and medial-lateral condyle motion, to constantly change the instantaneous centers of rotation during flexion. Such means may include means to guide the medial condyle of the femoral cap during flexion in a substantially anterior-posterior direction through a curved articular surface of the tibial plateau involving a moving point of contact between the lateral condyle of the femoral cap and a pivotal articular surface of the tibial plateau while rotating and translating the femur-tibia in the sagittal plane. These means may be provided by extension surfaces in the tibial plateau mated at extension with the condyles of the femoral cap with maximal surface contact, flexion surfaces mated during flexion with the condyles with substantial surface contact, and upwardly extending guidingbearing surfaces about these mating surfaces for guiding the movement during flexion and preventing dislocation. Preferably, the tibial plateau is constructed of an inert molded high-density plastic, such as high density polyethylene.

The mated tibial plateau and femoral cap combination of the present invention may be obtained by: (a) forming a substantially life-size femoral cap and cutting at least one-eighth inch from the outer surfaces of the cap to form a substantially smooth and round femoral cap having medial and lateral condyles', (b) pressing the femoral cap into soft molding material to initiate the formation of a tibial plateau having medial and lateral mating surfaces, the tibial plateau thereby being provided with extension surfaces and upwardly extending guiding-bearing surfaces about the extension surfaces; (c) permitting the molding material to harden sufficiently to fix substantially the extension surfaces but insufficiently to preclude further modification; and (d) continuously rotating and translating the femoral cap and tibial plateau through full flexion while substantially constantly changing the instantaneous centers of rotation and while pivoting by anterior-posterior motion the medial condyle about the lateral condyle, thereby forming flexion surfaces on the tibial plateau and upwardly extending guiding-bearing surfaces around both the extension and flexion surfaces.

If desired, a method may be used for forming a mated tibial plateau and femoral cap combination, which is characterized by significantly reduced friction and wear during use. Such combination is obtained by rotating and translating the femoral cap upon the tibial plateau in a sagittal plane through flexion, while applying compressive forces between the femoral cap and tibial plateau in cyclical loading to conform with the basic determinates of gait, and while pivoting in a transverse plane the medial condyle of the femoral cap through a curved articular surface of a medial mating surface of the tibial plateau, about a portion of the lateral condyle in contact with a lateral mating surface of the tibial plateau. The rate of erosion of the tibial plateau is measured during such movement, which is continued until the rate of erosion reaches a substantially constant minimum. Preferably, this motion is carried out while the joint is in an artificial environment simulating synovial fluid, for example in bovine serum.

In general, prostheses of this invention comprise a male component constructed of smooth, hard metal al loy, and a mated, molded, female component of plastic having mated flexion and guiding-bearing surfaces. Such prostheses may be formed by molding the female component in contact with the male component in fixed position, followed by passing the joint through flexion, in order to form articular surfaces upon the female component.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood by reference to the attached drawings, which show knee joint and elbow prostheses, and wherein:

FIGS. la and 1b are perspective views of the knee prosthesis combination during full flexion, FIG. 1a being a view of the tibial plateau component, and FIG. lb being a view of the femoral cap component, viewed from above and to the rear of the prosthesis;

FIGS. 2a and 2b are views in perspective of the knee prosthesis, FIG. 2a being the tibial plateau component and FIG. 2b being the femoral cap component, in full flexion, viewed from above and to the front of the prosthesis;

FIG. 3 is a further view of the combination in flexion illustrated in FIGS. la, b and 2a, b, showing in perspective the two components together, as viewed from the medial side, slightly in front of the prosthesis, and also showing the position of the femur, tibia and fibula bones of the leg;

FIG. 4 is a view of the prosthesis in extension viewed from the same position as FIG. 3;

FIGS. 5a and 5b are plan views of the knee prosthesis components, respectively the femoral cap (a) and the tibial plateau (b), viewed along the axis of the femur and tibia, showing by relief lines the topographical features of the articular surfaces of the components;

FIGS. 6a and 6b are views like FIGS. 5a and 5b, without the relief lines, showing the outer edges of the extension surfaces in contact with the components at extension;

FIGS. 7a and 7b are views like FIGS. 5a and 5b, without the relief lines, showing the outer edges of the flexion surfaces in contact with the components at 45 degree flexion;

FIGS. 8a and 8B are views like FIGS. 5a and 5b, without relief lines, showing the outer edges of the flexion surfaces in contact with the components at 90 degree flexion;

FIGS. 9a and 9b are views like FIGS. 5a and 5b, without the relief lines, showing the outer edges of the flexion surfaces in contact with the components at 135 degree flexion;

FIG. 10 is a perspective view from the same point as FIG. 1a, of the tibial plateau component with a portion cut away to provide space for retention of the posterior cruciate of the joint;

FIGS. 11a and 11b are views in perspective of the components of an elbow joint prosthesis, including a humeral cap (a) and an ulnar component (b), both viewed from a point in front of the joint slightly to the medial side; and

FIG. 12 is a view of the elbow joint components together with the joint flexed at an angle of about 135, viewed from the medial side above and behind the joint.

DETAILED DESCRIPTION OF THE INVENTION functions of collateral or internal ligaments, and, de-

pending on the joint function, optionally an extension surface for mating with the male portion at extension.

The invention in one embodiment comprises a knee joint prosthesis consisting of two components, a femoral cap and a tibial plateau, which are mated and operable in conjunction to simulate the natural movement of a knee joint. The mating of the two components may be achieved by forming the femoral cap and employing it for the formation of the tibial plateau, by making an initial impression with the components in the extension position, and then by forming flexion surfaces on the tibial plateau by passing the combination through full flexion. In another embodiment, the invention comprises an elbow joint consisting of a humeral cap and an ulnar component, which are mated and operable in conjunction to simulate the natural joint movement. In

further embodiments, the invention comprises analogous components, which in conjunction operate as the normal ankle and finger joints, or the like.

FIGS. 1a and lb illustrate respectively the tibial plateau component and the femoral cap component of a knee prosthesis, viewed from above and to the rear, with the two components inthe position of full flexion. The embodiment illustrates is for replacement of the left knee joint. A right knee joint, of course, would be like that shown in the drawings, in mirror image, with the medial and lateral components reversed.

In FIG. la, tibial plateau 20 includes lateral articular surface 21 and medial articular surface 22, and therebetween raised surface 23 (which results from mating with the empty trochlear surface of the femoral cap, as hereinafter explained). As used herein, articular surface" refers to that part of the surface of a component of the prosthesis which during normal joint movement comes into contact with another component. Thus, in the case of the tibial plateau 20, its articular surfaces (21,22) are those portions which contact the femoral cap. Articular surfaces 21 and 22 include upwardly extending guiding-bearing surfaces (lateral) 24 and upwardly extending guiding-bearing surfaces (medial) 25. Within guiding-bearing surfaces 24 and 25 are extension surfaces (lateral) 26 and (medial) 27. Between guiding-bearing surfaces 24 and 25 and extension surfaces 26 and 27, respectively, are flexion surfaces (lateral) 28 and (medial) 29. As will be seen more clearly in later FIGS. (6-9), the extension, flexion and guidingbearing surfaces (24-29) overlap and coincide at certain points.

FIG. lb illustrates femoral cap 30, comprising lateral condyle 31, medial condyle 32, intramedullary stem 33 and trochlear surface 34. Tibial plateau 20 and femoral cap 30 are also shown in FIGS. 2a and 2b, from a different view, in front of and above, also in the position of full flexion. As shown best in FIG. 2b, femoral cap 30 comprises extension surfaces (lateral) 35 and (medial) 36, as well as flexion surfaces (lateral) 37 and (medial) 38).

Tibial plateau 20 and femoral cap 30 are shown together, in a flexion position, in FIG. 3. In this view, the leg bones are also shown, namely femur 14, tibia 15 and fibula 16. The cement, e.g. methyl methacrylate 17, used to seat the tibial plateau 20, is also shown, as well as screws 18. Femoral cap 30 contacts the flexion surfaces 28 and 29 of tibial plateau 20 at two or more lateral-medial spaced substantial points, which lend stability to the joint during flexion. FIG. 4 illustrates the two components in the position of extension, in which position three or more lateral medial and anteriorposterior spaced substantial points of contact exist between the opposed surfaces, whereby the joint is effectively locked in extension (see also FIGS. 6a and 6b).

By the term substantial points of contact I mean to describe the contact between two mating curved surfaces, the male member of which is slightly smaller in curvature. In theory a point or line contact is made, but in practice when at least one member is resilient, more than a point or line contact results under pressure. The principle of multiple point (more than two) or maximal surface contact at extension combined with substantial surface (one, two or a line) contact at any point during flexion is a unique and important aspect of the present prosthesis, and is in part responsible for its capability of functioning like the normal knee joint. Preferably, this maximal surface contact includes substantial points distributed across at least 75 percent of the articular surface of the tibial plateau. The maximal surface contact at extension between the tibial plateau and femoral cap permits a strong locking of the two components in this position. The substantial surface contact, essentially contact (at two or more points, medial and lateral) across a substantial area, facilitates movement through flexiona simultaneous rotation of the femur with respect to the tibia and pivoting of the medial condyle with respect to the lateral condyle. These substantial points or lines of contact continually shift during flexion, and the overall flexion surfaces of the tibial plateau and femoral cap, which include all of the points of contact throughout flexion, due to the slight resilience of the plastic portion, are of considerable area when subjected to pressure. As a result, wear is spread over a large area of both mating surfaces, and no one spot or position is continuously subjected to wear. The biomechanical expression of this phenomenon is that the instantaneous centers of rotation are constantly changing during flexion. Moreover, the surface areas not in contact at any single position of flexioni.e. extension or guiding-bearing surfaces, or adjacent flexion surface-are very close, although not in direct contact, and form a very small angle, which orientation tends to equalize pressure of the joint upon the synovial fluid in the capsule and to further reduce friction and wear during use.

During flexion, when the femure is rotated with respect to the tibia, at any angular position between the two extremes (of l3S") there is a preferred set of sub stantial points (or lines) of contact between each of the two condyles and the two mating surfaces of the tibial plateau; this is a kind of preferred bottom point where the condyle rests. The muscles that traverse the knee have a built-in pattern that matches the contour and configuration of the tibial portion in flexion, extension and rotation. The function of the tibial portion of the present knee prosthesis is to act as a guide. The forces which act during extension tend to compress both sides of the joint together at any position and cause the two components to come together where the forces are least.

A further advantage of the present device is that it may absorb extremely high forces, which can exist for example during running movement upstairs and may reach as high as 3,000 pounds. The preferred composition for the tibial component is high-density polyethelene, which is to a limited degree resilient. Therefore, when excessive forces are applied at the points of contact between the femoral cap and tibial plateau, the force will be distributed over an enlarged point due to the compression of the flexion surface portion of the tibial plateau. Again, the relatively small angles that exist between the articular femoral-tibia surfaces not in direct contact also supplement this resiliance of the contacted surfaces to obtain a synergistic result.

The shape of the two components is also important. The femoral cap component is similar to the distal end of the natural femur, except for being smaller all around and for the fact that the lateral surfaces are cut back at an angle. About one-eighth to one-fourth inch is cut off around the entire femoral cap from both condyles; in addition the side portions, lateral surface 39 and medial surface 40 of the femoral cap, are cut back (FIGS. 1b, 2b, 3 and 4). This shaping of the femoral cap, and cutting back, permits the cap to seat within the upwardly extending guiding-bearing surfaces 24 and 25 of the tibial plateau 20 for locking purposes at extension (FIG. 4) and for stability during flexion (FIG. 3). An additional reason for making the femoral component smaller than life-size is simply to reduce the size of the joint for reasons of appearance, possibly for comfort of the patient, and also possibly to avoid enlarging the joint in such a way as to increase the chance of bumping it in use. Preferably, the main curves of the condyles 31 and 32 of femoral cap 30 are circular, in order to improve uniformity during the formation steps described hereinafter. Thus the preferred femoral cap component 30 is similar in shape to a natural femoral cap, but is smaller and has its lateral and medial surfaces 39 and 40 cut back, and is shaped to give circular main curves to the condyles 31 and 32.

The tibial plateau component 20, however, is unique and quite different from the proximal end of the tibia found in nature. This difference is in part due to the fact that the tibial plateau 20 usually must serve the functions of the anterior and posterior cruciates as well as the menisci in the normal knee. These functions are accomplished in the present prosthesis. by raised mating surfaces, i.e. upwardly extending guiding-bearing surfaces 24 and 25, over which the femoral cap 30 must ride if it is to shift out of proper position. These raised surfaces 24 and 25 do not exist in nature, but in nature the cruciates and menisci serve to limit such motion in the joint. In certain respects, the present prosthesis is better than nature because the cruciates and menisci can fail during use, while comparable failure of the elements in the tibial plateau of the present prosthesis is extremely unlikely.

FIGS. 5a and 5b show femoral cap 30 and tibial pla teau 20, viewed axially, with relief lines 41-46, and 51-54, showing the topographical features of the articular surfaces of the components. These relief lines are not intended to be sufficiently precise to duplicate but rather are intended to delineate the general shaping of the articular surfaces. FIGS. 60 and 6b show the outer boundaries of the extension surfaces 60 and 61 of the two components (it should be noted that certain portions, relatively small in area, within the boundaries are not actually in contact, but this is not shown, for simplicity in presentation). FIGS. 7a, 7b, 8a, 8b, 9a and 9b show the boundaries of the flexion surfaces at various degrees of flexion, including 45 degrees (62 and 63 of FIGS. 7a and 7b); (64 and 65 of FIGS. 80 and 8b); and degrees (66 and 67 of FIGS. 9a and 9b). Portions of the flexion surfaces of the femoral cap 30 are not shown, because of the axial view (i.e. parts of 62, 64 and 67). It is notable that the substantial points of contact of the articular surfaces may include two, three or four, (not shown) points or even lines, covering a very substantial area to provide joint stability. By way of contrast, femur-tibia contact of the normal knee joint, through the menisci, occurs at two small points only, covering a far smaller area.

It is a characteristic of the flexion surfaces that they diverge through flexion, cf. 63,65 and 67, due to the anterior-posterior divergence of the femoral condyles. Thus, while the initial (no degree) flexion surfaces (62 and 63) lie at least in part within the extension surfaces (60 and 61), the final 135) flexion surfaces (66 and 67) lie completely without.

FIG. 10 illustrates an embodiment of tibial plateau having an open notch 70 (removed from raised surface 23) to provide for retention of the posterior cruciate ligament (not shown), which would connect the tibia and the femur at a point posterior to the trochlear surface 34 of femoral cap (see FIG. 2b).

Thus in the present knee prosthesis, the function of the cruciates and menisci is performed preferably by the combination of lateral and medial extension surfaces, flexion surfaces and guiding-bearing surfaces. It should be noted that the centers of contact of the extension surfaces and 36 of the femoral cap 30 (60 in FIG. 6a) are relatively close together, and as the joint passes through full flexion the points of contact diverge continuously until full flexion is reached, where the points of contact on the lateral and medial condyles 31 and 32 (66 in FIG. 9a) have reached a maximum. Thus it is a characteristic of the tibial plateau that the extension surfaces 26 and 27 lie predominantly inside the flexion surfaces 28 and 29 and the upwardly extending guiding-bearing surfaces 24 and 2S surround both the extension surfaces and the flexion surfaces. Furthermore, since at virtually no point in any of the surfaces is a level region reached, and a curvature always exists, the upwardly extending guiding-bearing surfaces 24 and 25 actually include part of the extension surfaces 26 and 27 and flexion surfaces 28 and 29, and the three surfaces overlap and function cooperatively and interdependently.

The relative movement of the femur and tibia during flexion of the knee joint is a matter still of some controversy among doctors. In any event, the present knee prosthesis is the only one proposed to date that is capable of permitting natural knee movement. The movement is considered by the consensus of authorities to include (a) rotation in two planes: first a rotation of the tibia with respect to the femur in a sagittal plane, and second, a rotation of the femur in a transverse plane; and (b) translation in the sagittal plane.

For simplicity, herein, the first rotation, in the sagittal plane is referred to as rotation (arrow 10 in FIGS. 2a, 2b and 4), while the second, in the transverse plane is referred to as pivoting (arrow 11 in FIGS. lb, 2b and 4). Thus, the movement of the tibia with respect to the femur (including translation) is therein called rotation," while the turning of the femur in the transverse plane is referred to as a pivoting of the medial condyle about the lateral condyle. In the action of the normal joint during full flexion, the former rotation passes through about 135, while the latter pivoting passes through 420(l(ettlecamp et al., J. Bone & Joint Surgery, Sept. 1970, p. 775, give the angle of rotation of this pivoting" as 13 on average, with a range of 6 to 20; Levens et al., J. Bone & Joint Surgery, Oct. 1948, p. 865, found an average of 9 and a range of 4 to 13.), depending upon the individual, shape of the components and other factors. In any event, the capability of the present prosthesis to perform both rotation and pivoting simultaneously during flexion is unique and is a very important characteristic of the device.

This rotation-pivoting motion has been referred to as the screw hole effect, in that the movement of the knee joint represents to a limited extent the movement of screw threads. Thus, as the joint flexes, the lateral articular surface of the tibial plateau mating with the lateral condyle of the femoral cap primarily acts as a pivot without shift of position anteriorly-posteriorly (a-p), while the medial condyle slides from a posterior position anteriorly in the medial articular surface of the tibial plateau. This affects the shape of the medial and lateral flexion surfaces of the tibial plateau. Thus, the lateral flexion surface 28 is roughly in the shape of a slightly elliptical spheroidal arc, since the lateral condyle 31 is preferably circular and rotates in a fairly constant position through 420. By comparison, the medial flexion surface 29 of the tibial plateau is considera bly lengthened, since it provides a channel for the movement of the medial condyle 32 as it pivots about the lateral condyle 31 during flexion.

As mentioned above the upwardly extending guidingbearing surfaces (lateral and medial) 24 and 25 of the tibial plateau play an important function. Not only are these surfaces a vital part of the extension surfaces 26 and 27 and flexion surfaces 28 and 29 of the tibial plateau, they also (to the extent that they extend above the outer limits of the extension and flexion surfaces) provide a measure of safety, should the flexion exceed normal motion, due to extreme force or the like. Furthermore, that part of the upwardly extending guidingbearing surfaces which forms the outer edges of the extension surfaces in the tibial plateau is vital to the locking capability of the present knee prosthesis.

One of the problems in the prior art knee prostheses has been wear and fatigue during operation due to excessive stress on too small an area of the supporting parts without adequate penetration of lubrication. This is particularly true for the hinged joints, where the joint necessarily flexes through a single plane, and where the natural tendency to flex through both the axial and transverse plane simultaneously puts extreme stress on the hinged joint. A major advantage of the present device is that a rotating-pivoting motion is possible during flexion, which reduces considerably the stress on the joints in useand permits penetration of synovial fluid into the wear receiving area. A further advantage of the present prosthesis is that, as the joint flexes, the bearing surface shifts from one position to another in the joint, on the flexion surfaces of the tibal plateau (i.e. 63, to 65, to 67). In this way, the wear and synovial fluid are spread over a large area of both of the mating surfaces, and no one spot is continuously getting all the wear. A further advantage is the fact that the supporting components in the mating surfaces rest on substantial points of contact, except in the locked, extension position. However, the difference during flexion between the radii of the flexion surfaces of the tibial plateau and femoral cap condyles at any point in flexion is very slight, and since the material which the tibial plateau is made is slightly resilient, it will never be an actual point contact but instead the pressure will be distributed in the joint over a widened area, which are increases directly with an increase in stress.

As mentioned briefly above, forms for the femoral cap and tibial plateau may be prepared by forming a substantially life-size femoral cap; cutting at least oneeighth-inch from the outer surfaces of said cap while forming substantially smooth and round medial and lateral condyles; pressing said femoral cap into molding material to initiate the formation of a tibial plateau having medial and lateral mating surfaces, whereby the tibial plateau is provided with extension surfaces mating said femoral cap with maximal surface contact and upwardly extending guiding-bearing surfaces about said extension surfaces; permitting said molding materialto harden sufficiently to fix substantially at least a portion of said extension surfaces but insufficiently to preclude further modification; and continuously rotating said femoral cap through full flexion while substantially constantly changing the instantaneous centers of rotation and while pivoting by anterior-posterior motion said medial condyle about said lateral condyle, whereby the tibial plateau is provided with flexion surfaces mating said femoral cap with minimal surface contact during flexion and whereby said upwardly extending guiding-bearing surfaces of said medial mating surface are lengthened anteriorly-posteriorly. Such forms may be mated further by passing a combination repeatedly through flexion by mechanically simulating natural movement, by rotating a femoral cap composed of hard attritive material and including substantially smooth and round medial and lateral condyles; in contact with a tibial plateau composed of hard but erodible material and including medial and lateral mating surfaces, each having an extension surface, a flexion surface and upwardly extending guiding-bearing surfaces; while applying compressive forces between the femoral cap and tibial plateau simulating relatively natural forces therebetween during flexion; and while pivoting the medial condyle through a curved articular surface of the medial mating surface about a portion of the lateral condyle in contact with the lateral mating surface; measuring the rate of erosion of the tibial plateau during said rotating and pivoting; and continuing the rotating and pivoting until the rate of erosion reaches a substantially constant minimum. These methods for forming and mating prosthesis components may be employed as well for joints other than the knee.

The surgical technique of implanting the present knee prosthesis is not substantially different from techniques used today for similar procedures lnitially, the cruciate ligements are cut (or, possibly, only the anterior cruciate), and the distal end of the femur and the proximal end of the tibia are amputated. The remaining collateral ligaments, muscles, patella and the like are preferably left intact. and as a result the joint when inserted cannot be pulled apart. Next, drill holes are made into the remainder of the tibia, which is then packed with a suitable cement material, such as methyl methacrylate, and the plastic tibial plateau is then set in the adhesive. Similarly, a hole is drilled into the femur for insertion of the intrameduallary stem, and the femoral cap is set into the distal end. Finally, the joint is assembled, and the remaining ligaments and the like are put in their proper place.

Much of the above discussion with respect to the knee prosthesis applies to the other embodiments of the invention, elbow, ankle and finger prostheses and the like. One such embodiment is shown in FIG. 11a, 11b and 12, wherein is shown an elbow joint prosthesis 71, (for the right arm), consisting of humeral cap 72 and ulnar component 73. Humerai cap 72 includes capatellum surface 74 (corresponding to the capatellum which in the natural joint joins with the radial head-normally amputated), and trochlear surface 75, both of which comprise overlapping extension surfaces 76 and flexion surfaces 77, and intramedullary stem 78. Ulnar component 73 includes coronoid process 79, olecrenon 80, olecrenon process 81 and radial head replacement 82, which together also comprise overlapping extension surfaces 83 and ilexion surfaces 84; upwardly extending guiding-bearing surfaces 85; and intramedullary stem 86, for insertion into the ulnar. It should be noted that in the present elbow prosthesis, the radial head is removed in surgery; its function preferably is performed by radial head replacement 82, forming part of ulnar component 73; and it is not connected at all to the radius but only to the ulna after insertion of the prosthesis. Similarly, that portion 87 of the humeral cap 72 which mates with the radial head replacement 82 need not be of the same shape as the analogous portion of the distal end of the humerus, but may be shaped, as shown, to improve the joint action and stability, for which purpose a humerus-radius joint is not essential.

There are several advantages of the present elbow prosthesis over a hinged joint. First, the joint permits the normal adduction and abduction (about 9) of the ulna upon the trochlea of the humeral cap 72, which exists in normal motion, and also upon the capatellum surface 74. Also, greater stability is provided on the capatellum surface, which after most cases of elbow surgery is not in contact with anything due to the ab' sence of the radial head. Finally, the prosthesis permits the slight transverse movement of the ulna during flexion, due to the screw thread" nature of the raised sur faces (three) of the humeral cap.

I claim:

1. A prosthesis for a human joint for replacement of amputated articulating surfaces thereof and adapted for articulating movement between extreme positions of extension and flexion comprising, a smooth rounded male portion having a continuous convex flexion surface defining a compression receiving area, a female portion shaped to provide a continuous curved elongated concave surface including a flexion surface at the bottom of said concavity and guide bearing surfaces laterally thereof adapted to receive said male portion and defining a compression receiving area at the flex ion surface of said female portion, said respective compression receiving areas being in contact only at a substantial point of contact at which the principal forces across said joint are concentrated, said male portion being free to rotate on and slide along the i'lexion and guide bearing surfaces of said female portion, and the respective positions and curvatures of said flexion surfaces comprising means responsive to the combined vector forces surrounding said joint after said amputation for continuously longitudinally shifting said substantial point of contact during flexion.

2. The prosthesis according to claim 1 further including means comprising an extension surface on said female portion for locking said human joint at extension.

3. The prosthesis according to claim 2 wherein said human joint is a human knee joint, said male portion is a femoral cap having medial and lateral condyles, said male portion is a mated tibial plateau and further including means responsive to the combined vector forces surrounding said knee joint after said amputation to guide said medial condyle during flexion in a substantially anterior-posterior direction through a curved articular surface of said tibia plateau about a pivot point of contact between said lateral condyle and a pivotal articular surface of said tibial plateau and for forcing fluid therebetween under dynamic fluid conditions to lubricate said maximum compression areas.

4. The prosthesis according to claim 1 wherein said male portion is constructed of an inert metal alloy and said female portion is constructed of an inert molded high-density plastic.

5. A knee prosthesis comprising a femoral cap having medial and lateral condyles and a tibial plateau having medial and lateral mating surfaces for mating with said medial and lateral condyles of said femoral cap, wherein both tibial plateau surfaces contain an extension surface for mating at extension with said femoral cap with maximal surface contact and a flexion surface for mating during flexion with said femoral cap with substantial surface contact, said femoral cap further comprises at least one curved bearing surface having a constant or slightly continuously changing curvature radius for contact with said flexion surface of said tibial plateau, and wherein said flexion surface has a radius of curvature slightly greater than said curvature radius of said curved bearing surface at any given point of contact.

6. A knee prosthesis comprising a femoral cap having medial and lateral condyles and a tibial plateau having medial and lateral mating surfaces for mating with said medial and lateral condyles of said femoral cap, wherein both tibial plateau surfaces contain an extension surface, said extension surface including means for mating at extension with said femoral cap with maximal surface contact and a flexion surface, said flexion surface including means for mating during flexion with said femoral cap with less than maximal but still substantial surface contact, the location of said substantial surface contact continuously shifting during flexion.

7. The knee prosthesis of claim 6, wherein said maximal surface contact includes contact between the femoral cap and at least percent of said medial and lateral mating surfaces of said tibial plateau.

8. The knee prosthesis of claim 6, wherein said tibial plateau comprises upwardly extending guiding-bearing surfaces about said medial and lateral mating surfaces.

9. The knee prosthesis of claim 6, wherein said femoral cap is constructed of an inert metal alloy and said tibial plateau is constructed of an inert molded highdensity plastic.

10. A knee prosthesis comprising a femoral cap having medial and lateral condyles and a tibial plateau having medial and lateral mating surfaces for mating with said medial and lateral condyles of said femoral cap, wherein both tibial plateau surfaces contain an extension surface for mating at extension with said femoral cap with maximal surface contact and a flexion surface for mating during flexion with said'femoral cap with substantial surface contact, said prosthesis further includes means to guide said medial condyle during flexion in a substantial anterior posterior direction through a curved articular surface of said tibial plateau about a pivot point of contact between said lateral condyle and a pivotal articular surface of said tibial plateau. 

1. A prosthesis for a human joint for replacement of amputated articulating surfaces thereof and adapted for articulating movement between extreme positions of extension and flexion comprising, a smooth rounded male portion having a continuous convex flexion surface defining a compression receiving area, a female portion shaped to provide a continuous curved elongated concave surface including a flexion surface at the bottom of said concavity and guide bearing surfaces laterally thereof adapted to receive said male portion and defining a compression receiving area at the flexion surface of said female portion, said respective compression receiving areas being in contact only at a substantial point of contact at which the principal forces across said joint are concentrated, said male portion being free to rotate on and slide along the flexiOn and guide bearing surfaces of said female portion, and the respective positions and curvatures of said flexion surfaces comprising means responsive to the combined vector forces surrounding said joint after said amputation for continuously longitudinally shifting said substantial point of contact during flexion.
 2. The prosthesis according to claim 1 further including means comprising an extension surface on said female portion for locking said human joint at extension.
 3. The prosthesis according to claim 2 wherein said human joint is a human knee joint, said male portion is a femoral cap having medial and lateral condyles, said male portion is a mated tibial plateau and further including means responsive to the combined vector forces surrounding said knee joint after said amputation to guide said medial condyle during flexion in a substantially anterior-posterior direction through a curved articular surface of said tibia plateau about a pivot point of contact between said lateral condyle and a pivotal articular surface of said tibial plateau and for forcing fluid therebetween under dynamic fluid conditions to lubricate said maximum compression areas.
 4. The prosthesis according to claim 1 wherein said male portion is constructed of an inert metal alloy and said female portion is constructed of an inert molded high-density plastic.
 5. A knee prosthesis comprising a femoral cap having medial and lateral condyles and a tibial plateau having medial and lateral mating surfaces for mating with said medial and lateral condyles of said femoral cap, wherein both tibial plateau surfaces contain an extension surface for mating at extension with said femoral cap with maximal surface contact and a flexion surface for mating during flexion with said femoral cap with substantial surface contact, said femoral cap further comprises at least one curved bearing surface having a constant or slightly continuously -changing curvature radius for contact with said flexion surface of said tibial plateau, and wherein said flexion surface has a radius of curvature slightly greater than said curvature radius of said curved bearing surface at any given point of contact.
 6. A knee prosthesis comprising a femoral cap having medial and lateral condyles and a tibial plateau having medial and lateral mating surfaces for mating with said medial and lateral condyles of said femoral cap, wherein both tibial plateau surfaces contain an extension surface, said extension surface including means for mating at extension with said femoral cap with maximal surface contact and a flexion surface, said flexion surface including means for mating during flexion with said femoral cap with less than maximal but still substantial surface contact, the location of said substantial surface contact continuously shifting during flexion.
 7. The knee prosthesis of claim 6, wherein said maximal surface contact includes contact between the femoral cap and at least 75 percent of said medial and lateral mating surfaces of said tibial plateau.
 8. The knee prosthesis of claim 6, wherein said tibial plateau comprises upwardly extending guiding-bearing surfaces about said medial and lateral mating surfaces.
 9. The knee prosthesis of claim 6, wherein said femoral cap is constructed of an inert metal alloy and said tibial plateau is constructed of an inert molded high-density plastic.
 10. A knee prosthesis comprising a femoral cap having medial and lateral condyles and a tibial plateau having medial and lateral mating surfaces for mating with said medial and lateral condyles of said femoral cap, wherein both tibial plateau surfaces contain an extension surface for mating at extension with said femoral cap with maximal surface contact and a flexion surface for mating during flexion with said femoral cap with substantial surface contact, said prosthesis further includes means to guide said medial condyle during flexion in a substantial anterior -posterior direction through a curved articular surface of said tibiAl plateau about a pivot point of contact between said lateral condyle and a pivotal articular surface of said tibial plateau. 